1. Field of the Invention
The field of art to which this invention pertains is the separation of solids from fluids, and more particularly, the separation of hydrocarbon oils from catalyst fine materials in a slurry settler specifically used in fluidized catalytic cracking processing of petroleum.
2. Description of the Prior Art
Several methods are currently available to control and reduce fluidized catalytic reaction unit regenerator stack catalyst particulate emissions to the atmosphere. These include the use of electrostatic precipitators, high efficiency external cyclones, and various dip leg withdrawal schemes. These devices are generally installed to recover catalyst particles ranging in size from about 0 to about 20 microns diameter. The efficiency of conventional internal regenerator cyclones is presently very high on large particles but in many instances suffers when trying to remove small particles (less than 20 microns diameter) from the regenerator off-gas vented to the atmosphere.
A well known alternate method to control rejection of catalyst fines in the regenerator effluent gas is to allow catalyst fines to escape from the reactor cyclones and pass into the main column or fractionation system. If 0 to 20 microns catalyst particles are rejected from the reactor-fractionator system, then losses of this size range material from the regenerator stack will be reduced, if not totally eliminated, since most of the catalyst fines are eventually removed from the process. This method of operation is quite common in the industry, being practiced by many refiners. Typically, to implement such a processing scheme, the fractionator bottoms slurry stream is withdrawn from the slurry setter and sent to tankage, with removal of the settled catalyst at scheduled intervals. The clarified oil from such settling tanks can be recycled to the catalytic cracking reaction zone or other processing.
Unfortunately, if reactor cyclone efficiencies are poor either by virtue of design or for mechanical reasons, the reactor catalyst losses are intolerably high and rejection of particles appreciably larger than 20 microns occurs. Many catalytic cracking units are therefore equipped with slurry settling vessels to recover and recycle catalyst back into the reactor system.
The present inventive concept allows the use of a slurry settling zone but requires that at least a portion of the decanted oil from an uppermost portion of this slurry settling zone be eventually recycled to a lower portion of the slurry settling zone under some control mechanism to regulate the upward velocity of the oil in the slurry settling zone to selectively remove the catalyst particles of less than about 20 microns diameter. By selectively regulating the upward velocity of oil in the slurry settler by the recycle of decanted or clarified oil to the slurry settler, the size of catalyst particles being removed from the system can be closely regulated. This will minimize stack emissions frm the fluidized catalytic cracking regeneration effluent gases while also minimizing the total catalyst loss from the overall process.
U.S. Pat. No. 2,879,224 to S. D. Lawson, which issued Mar. 24, 1959, is considered to be the most relevant art that Applicants and their attorney have reviewed. This patent relates to the separation of solids from fluids in a settling zone and requires that a low-coking fluid displace a high-coking fluid in a catalyst fines slurry mixture which mixture is then recycled to a fluidized catalytic cracking unit. A fluid is pumped into the settling zone at a regulated rate to allow the displacement of the high-coking fluid through the use of the low-coking fluid while still allowing settling, in a downward direction, of catalyst particles which are ultimately recycled to the cracking reaction zone.